An MGH-led study of Mongolian schoolchildren supports the possibility that daily vitamin D supplementation can reduce the risk of respiratory infections in winter. In a report that will appear in the September Pediatrics and is now available online, an international research team found that vitamin D supplementation decreased the risk of respiratory infections among children who had low blood levels of vitamin D at the start of the study.

“Our randomized controlled trial shows that vitamin D has important effects on infection risk,” says MGH Emergency Services physician Carlos Camargo, MD, the study’s corresponding author. “In almost 250 children with low blood levels of vitamin D during winter, we found that taking a daily vitamin D supplement cut in half the risk of a respiratory infection.”

Several recent investigations have suggested that vitamin D – best known for its role in the development and maintenance of strong bones – has additional important roles, including in immune function. Since vitamin D is naturally produced by the body in response to sunlight, maintaining adequate levels in winter is particularly challenging in areas such as the northern U.S. and Canada, which have significant seasonal variations in daily sunlight.

The current study analyzed data from the Blue Sky Study conducted in Ulaanbaatar, Mongolia, by a team led by Harvard investigators in collaboration with local health researchers. Mongolians are at high risk for vitamin D deficiency, especially during winter. The Blue Sky Study followed schoolchildren who had low blood levels of vitamin D at the study’s outset.

Camargo and colleagues compared the number of winter respiratory infections among a group of children who received daily doses of vitamin D added to locally produced milk with that of a control group receiving the same milk without added vitamin D. Based on reports from their parents, the children receiving vitamin D had about half the incidence of respiratory infections that the control group had.

“Our study provides strong evidence that the association between low vitamin D and respiratory infections is causal and that treating low vitamin D levels in children with an inexpensive and safe supplement will prevent some respiratory infections,” says Camargo. “The large benefit was undoubtedly related to the low baseline vitamin D levels of these children, so I would not expect the supplement to provide similar benefit in children who start with healthy levels of vitamin D. The key question for future research is at what initial vitamin D level would children no longer receive benefit from winter supplementation?”

The study’s authors also point out that, while Mongolia may appear to have little in common with the U.S., the low baseline vitamin D levels seen in study participants are relatively common in some groups of Americans, such as African-American children living in northern states.